There's a theory called the "diathesis stress model" that says genetic predisposition and environmental stress are involved in the development of mental illnesses. If the combination of genes and stress exceed a threshold, then you developed issues. So this theory says you can have a clear genetic predisposition but if you have minimal stress (and stress can be a lot of things--traumatic events, physical illness, neglect, abuse etc), you won't develope MI. Or you can have a minor genetic predisposition but if you have a whopping dose of stress or repeated stressful events, you'll develop MI. But you have to have both.
Interesting theory generally supported by twin studies.